Democrat John F. Street and Republican Sam Katz took their act on the road again yesterday - this time to the studios of KYW-AM (1060), where they traded barbs and opinions on a familiar litany of issues ranging from taxes to school reform.

Their hour-long confrontation, broadcast during morning rush hour, was the latest chapter in what has become perhaps the most extensive mayoral debate in modern Philadelphia history.

With three weeks left before the election, Street and Katz have engaged in more debates and joint forums than any other Democratic and Republican pair in memory.

"Already they have exceeded [other campaigns] by a significant margin," said Frederick L. Voigt, executive director of the nonpartisan Committee of Seventy, an election-oversight group.

Street and Katz debated higher education issues last Friday at Temple University. Their previous encounters have included debates Oct. 2 at a TV studio and a theater and face-offs at community forums in Center City and Society Hill last month. The two are expected to debate at least twice more this week - and several more times before the Nov. 2 election.

In 1987, when Democratic Mayor W. Wilson Goode sought reelection in a battle with former Mayor Frank L. Rizzo, the two met just once - in a televised debate nearly a month before the election.

Joseph Egan, the 1991 Republican nominee, said yesterday he had face-to-face encounters with Democratic rival Edward Rendell "maybe five or six times."

Street and Katz both appear comfortable on their feet. Street gained debate experience in a long, difficult Democratic primary campaign last spring. Katz, who had no opposition in the Republican primary, had previously run for mayor and governor.

Goode suggested another reason for their willingness to face off.

"They are fighting for the same voters at this point - the swing voters," Goode said.

Political analysts say that while 500,000 Philadelphians may vote on Nov. 2, the outcome of the election could be determined by about 50,000 voters who could go either way.

These have been identified as white liberals - along with some middle-class blacks - who almost always vote for Democrats but who also feel attracted to Katz, a moderate former Democrat.

The broadcast session yesterday included a few testy comments between the candidates.

Street at one point accused Katz of putting forward "risky ideas" for school reform. Katz replied that Street had "no ideas," except for saying the schools ought to have more money.

"Sam Katz has some risky ideas about how he wants to improve the quality of public education," Street said. ". . .He supports the privatization of educational services, the likes of which we have seen around the country."

Katz responded: "That's a distortion."

Katz said he was for expanding the kind of thing the city school district is already doing - such as "purchasing services" from a Texas company, Community Education Partners, to provide schooling for violent and disruptive students.

Street, as in past debates, tried to pin down Katz on a proposal he had made while running for governor in 1994 - that state spending for schools ought to be reduced 12 percent.

Katz, also as in past debates, would not take the bait.

"Let's talk about my positions as a candidate for mayor," he said.

Said Street: "You're running away from your position."

Katz a moment later accused Street of using "fear tactics" in his campaign.

For several weeks, Street has been hammering at Katz to explain how he proposed to reduce the wage tax for city residents from 4.61 percent to 4 percent over four years without cutting city services.

Katz said yesterday he would explain how this week.

Clearly weary of the issue, Katz tried to refocus the tax debate on what he said were 19 votes by Street, as a member of City Council in the 1980s, to raise the wage tax and other taxes.

Street, he said, wants to "rewrite history" and focus only on his seven years as Council president, when he worked with Rendell to make modest cuts in wage and business taxes.

"In one night," Katz said, Street helped to raise the wage tax 15 percent by voting on Council to raise it to 4.96 percent for city residents, its peak.

Back and forth went that issue.

Street said that besides helping to create $1 billion in tax cuts along with Rendell, he worked at the tail-end of the Goode administration to oppose other tax increases.

In their television ads, the two have accused each other of being negative.

But both said yesterday they believed their give-and-take had focused on substance, not personality.

"We agree that we don't think this is nasty," Street said in the broadcast debate.

"We're talking about issues," Katz said. "We disagree on issues. My campaign is not engaged in nasty advertising."